The determination of fracture toughness of brittle materials is often conducted using three-point bend beam method, in which the initiating crack is usually in the form of a diamond-sawed notch. In this paper a model of the fracture process is proposed. The model can be used to predict the fracture toughness from the apparent toughness and microstructural parameters, where the apparent toughness is measured using the sawed-notch crack method. The dependence of K_IC on the notch root radius was examined in two kinds of polycrystalline alumina and one soda-lime glass for notch radii as small as a few micrometers, The results show that when the notch radius is near to or smaller than the average grain diameter, the toughness is almost constant. When the critical notch radius used for determining toughness is approximately equal to the average grain diameter, a true measure of fracture toughness can be obtained.